Ethernet (registered trademark), which has been developed as local area network (LAN) technology, is beginning to be applied to large-scale carrier networks. Conventionally, Ethernet (registered trademark) had few functions related to Operation, Administration and Maintenance (hereinafter, “OAM”) for large-scale networks; however, with increased use of Ethernet in carrier networks, there is a growth in demand for OAM functions. According to Ethernet Frame Loss Measurements (“ETH-LM”), which have been standardized by ITU-T SG 13 as Y.1731, the number of transmission packets or the number of reception packets is stored in an OAM frame (which is a monitoring and control Ethernet frame), and the OAM frame is then output to report the number to the other end of the communication.
Subsequently, the reported number of transmission packets in the OAM frame is compared with the number of actually received packets, or the reported number of reception packets in the OAM frame is compared with the number of actually transmitted packets. Herewith, loss of packets, that is, frame loss can be measured. There are two types of measurement methods for ETH-LM, Single-ended ETH-LM and Dual-ended ETH-LM.
These two methods differ in the type of OAM frames used, information stored in OAM frames and output, and the frame loss calculation method; however, in both the methods, when an OAM frame is transmitted, the count number (counter value) of packets (data frames) transmitted by the time when the OAM frame is transmitted is stored in the OAM frame.
Various studies of OAM functions for Ethernet (registered trademark) have been conducted. For example, Japanese Laid-open Patent Application Publication No. 2007-251541 discloses an invention in which when a transmitting side periodically transmits management frames to a destination side, the transmitting side specifies, in each management frame, a transmission cycle used on the transmitting side, thereby being able to adjust the transmission cycle of the management frame.
However, since data frames are not transmitted at regular temporal intervals, a subsequent data frame may be transmitted before the counter value is stored in an OAM frame and then transmitted. This leads to the problem that the counter value in the OAM frame disagrees with the number of data frames actually transmitted.
For example, assume the case in which there are many data frames to be transmitted and the data frames are transmitted in a continuous fashion.
For transmitting OAM frames, a method may be adopted in which an OAM frame including a counter value is made by referring to the counter value of transmitted data frames, and then output from an output port used also for outputting data frames.
In this case, if a data frame is transmitted before the output of the OAM frame, the counter value included in the OAM frame is smaller than the number of data frames transmitted by the time of the output of the OAM frame.
This problem can be prevented if transmission of the subsequent data frame is suspended until the OAM frame is output. However, the first priority should be placed on transmission of data frames that include user data, thus leaving a problem.